A: As far as it being legal, yes the program is legal but only if you are using it to download files that are not copyright protected. Let me rephrase that for emphasis, it is illegal to use file sharing programs to download copyrighted music, video, images, software, or files. 

As far as the program being harmful to your computer, well if you download a file that is from a virus or spyware infected computer that virus or spyware can be transferred to your computer. So yes the program can harm your computer. 

A: Technically you could get thousands upon thousands of free songs from music and file sharing programs, but those songs are pretty much guaranteed to have viruses. Those viruses can infect and crash your computer, not to mention that "free" music is completely illegal. Having Anti-Virus Software will not stop the viruses from entering your computer if you are using file sharing programs. Paying for the file sharing program will not make that music legal. You are paying for a legal program and using that legal program to illegally download music you are not paying for. In a nutshell, downloading copyrighted music/video/pictures is stealing. You are always better off using a service such as iTunes, Zune, Rhapsody, Napster, etc.